FIG. 1 shows a configuration example of a circuit (pixel circuit) for driving one pixel of an active type organic EL display device. A drain of a p-channel driving TFT 1 with a source connected to a source line PVdd is connected to an anode of an organic EL element 3, and a cathode of the organic EL element 3 is connected to a cathode source CV. A source of an n-channel selection TFT 2 is connected to a gate of the driving TFT 1, a drain of this selection TFT 2 is connected to a data line Data, and a gate is connected to a gate line Gate. Furthermore, one end of a holding capacitor C is connected to a gate of the driving TFT 1, and the other end is connected to a capacitor power source line Vsc.
Consequently, the data signal is accumulated in the holding capacitor C by making the gate line extending in the horizontal direction an H level, and switching on the selection TFT 2, and in this condition overlaying a data signal having a voltage corresponding to display brightness onto the data line Data which extends in the vertical direction. As a result, the driving TFT 1 supplies a driving current corresponding to the data signal to the organic EL element 3, so that the organic EL element 3 emits light.
Here, the light emission amount and the current of an OLED element are approximately proportionately related. Normally, a voltage (Vth) such that a drain current starts to flow in the vicinity of the black level of the image is applied between the PVdd and the gate of the driving TFT 1. Furthermore, an amplitude to give a predetermined brightness in the vicinity of the white level, is applied as the amplitude of the image signal.
FIG. 2 shows the relationship of current icv (corresponding to brightness) flowing in the organic EL element 3 with respect to voltage Vgs between the gate and source of the driving TFT 1 (the difference between the voltage of the data line Data and the power source PVdd). By determining a data signal such that Vth is applied as the black level voltage, and Va is applied as the white level voltage, appropriate tone control in the organic EL element 3 can be performed.
Here, the organic EL display device includes a display panel with a plurality of pixels arranged in matrix form. Therefore, as a result of problems in manufacture, Vth varies for each of the pixels, and even on one display panel, the optimum black level varies for each pixel. As a result, the light emission amount with respect to the data signal (input voltage) is non-uniform for each pixel, so that brightness unevenness occurs. Regarding this variation in Vth, cases where this changes randomly for each pixel are few, but there still are instances where this changes gradually over the whole display screen. In this case, even if the same voltage is input to all of the pixels, the brightness gradually changes as shown in FIG. 3. That is to say, in this example, in the x-direction, the right side is darker, and in the y-direction, the bottom side is darker. Consequently, this gives an image where the bottom right is dark and the top left is bright.
Moreover, in the case where the unevenness for each of the horizontal or vertical lines is significant, this appears as stripes for the respective directions.
It has also been proposed to measure the brightness of the respective pixels, and perform a correction for all of the pixels in accordance with the correction data stored in memory. See Japanese Patent Laid-Open No. Hei 11-282420.
However, in the method of this patent publication 1, there are problems in that brightness measurement is not easy with a display panel where there are a large number of pixels, and the capacity of the memory must be large. Furthermore, it is generally difficult to measure the brightness of the pixels with good accuracy and in a short time.